1. Field of the Invention
The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to a lithium cell suitable for current pulse discharge applications. More particularly, the present invention characterizes the shape of the first pulse during cell burn-in discharge or otherwise, to predict long-term performance. A particularly preferred electrochemical couple is a lithium/silver vanadium oxide cell (Li/SVO). The discharge characterization is especially important in the transition between the first and second voltage plateaus of a Li/SVO cell discharge where voltage delay typically occurs.
2. Prior Art
Efforts have been made to develop a test administered at the beginning of a cell's life that will be indicative of its long-term performance. Such a test would be useful as a means of screening out poor performers, problem solve root causes to various performance issues, and determine and identify the impact of certain factors or changes in components and manufacturing processes. Conventional methods include subjecting the cells to elevated temperature storage or Arhennius type accelerations, or comparison of individual cell burn-in data to the general population.
For example, U.S. Pat. No. 5,616,429 to Klementowski teaches conditioning a lithium/silver vanadium oxide cell by heating it at an elevated temperature of at least about 75° C. for an extended period of time longer than seven days. This is done so that when the cell is later subjected to a current pulse application, it will be at a high pulse rate without appreciable voltage delay. To show that such conditioning has its intended benefit, Klementowski subjected a test matrix of 132 Li/SVO cells to either an accelerated pulse test (APT), a 1-year accelerated discharge data (ADD) test, a 3-year ADD test or a 5-year ADD test. However, as stated at column 5, line 38 to column 6, line 5 of this patent, prior to pulse discharge testing, “all cells were subjected to a standard burn-in which consists of a 2.49 KΩ load for 17 hours, followed by an open circuit ‘rest’ period and a single pulse train of four, 2.0 amp pulses; 10 seconds on and 15 seconds open circuit per pulse at one week after elevated temperature conditioning.” It is known that burn-in discharge typically depletes a cell of about 2% of its total capacity. This means that every cell Klementowski used to determine what the minimum conditioning temperature must be and for what time duration to have a beneficial effect on subsequent pulse discharging was first depleted of about 2% of its total capacity.
On the other hand, the present invention consists of calculating the Plast−Pmin for a subject cell in comparison to that for a reference cell, both pulse discharged to deliver a first pulse or pulse train at the very beginning of their lives when they are new or freshly built cells. This is in contrast to depleting about 2% of the cell's total discharge capacity during burn-in before performing further APT or ADD tests as taught by Klementowski. Instead, it is desirable when the cell is essentially at the beginning of its discharge life (about 0% to about 1% DoD) to determine whether it will experience increased voltage delay under current pulse discharge conditions at some later stage in its depth of discharge in comparison to a second cell. This means that it is within the scope of the invention to have the cell experience some discharge, whether intended or self-discharge, prior to performing the calculation.
Furthermore, elevated temperature type tests introduce unknown or undefined reactions that may invalidate the correlation. Also, these tests often take months to produce a relationship and cannot be used for early screening purposes. Comparison of individual burn-in data to the general population can be used as a screen for gross performance issues, but is unable to differentiate between poor and good performers under long-term discharge, especially in the voltage delay region typically seen with SVO chemistry.
Prior work also shows a lack of correlation between an initial pulse minimum voltage (Pmin) or open current voltage (OCV) values and long-term performance. Many factors contribute to variation in burn-in results that confound and prevent developing a definitive link between initial and long-term performance. Higher initial pulse voltages do not necessarily indicate better long-term performance.
Therefore, there is a need for a test that is relatively easy to administer and evaluate and that differentiates between cells prone to experiencing unacceptable voltage delay and those that will not.